Many industrial applications require large scale generation of heat from burners for process heaters, boilers, or other fired heating systems. If the burner fuel is thoroughly mixed with air and combustion occurs under ideal conditions, the resulting combustion products are primarily carbon dioxide and water vapor. However, when the fuel is burned under less than ideal conditions, such as in a high temperature environment, nitrogen present in the combustion air reacts with oxygen to produce nitrogen oxides (NOx). It is well known that, other conditions being equal, NOx production increases as the temperature of the combustion process increases. NOx emissions are generally considered to contribute to ozone depletion and other environmental problems.
For gaseous fuels with no fuel bound nitrogen, thermal NOx is the primary mechanism of NOx production. Thermal NOx is produced when the flame reaches a high enough temperature to break the covalent N2 bond apart and the resulting “free” nitrogen atoms then bond with oxygen to form NOx.
Combustion air is comprised of approximately 21% O2 and 79% N2. Combustion occurs when the O2 reacts and is combined with the fuel (typically hydrocarbon). The temperature of combustion is not normally great enough to break all of the N2 bonds, so most of the nitrogen in the air stream passes through the combustion process and remains as diatomic nitrogen (N2) in the combustion products.
However, some of the N2 reaches high enough temperatures in the high intensity regions of the flame to break apart and form “free” nitrogen. Once the covalent nitrogen bond is broken, the “free” nitrogen is available to bond with other atoms. The free nitrogen, or nitrogen radicals, will react with any other atoms or molecules suitable for reaction. Fortunately, the free nitrogen will most likely react with other free nitrogen to form N2. However, if another free nitrogen atom is not available, the free nitrogen will react with oxygen to form NOx.
As the flame temperature increases, the stability of the N2 covalent bond decreases, allowing the formation of more and more free nitrogen and subsequently increased thermal NOx. Burner designs can reduce NOx emissions by reducing the peak flame temperature which in turn reduces the formation of free nitrogen available to form NOx.
The varied requirements of refining and petrochemical processes require the use of numerous types and configurations of burners. The method utilized to lower NOx emissions can differ from application to application. However, thermal NOx reduction is generally achieved by delaying the rate of combustion. Since the combustion process is a reaction between oxygen and a fuel, the objective of delayed combustion is to reduce the rate at which the fuel and oxygen mix together and burn. The faster the oxygen and the fuel gas mix together, the faster the rate of combustion and the higher the peak flame temperature.
NOx emissions increase as the adiabatic flame temperature increases. Slowing the combustion reaction allows the flame temperature to be reduced, and as the flame temperature is reduced, so are the thermal NOx emissions.
One of the best methods of thermal NOx reduction is to mix the fuel gas together with the inert products of combustion before combustion occurs, thus reconditioning the fuel. Since the new mixture is comprised of inert components, the resulting composition burns at a lower peak temperature.